1. Field of the Invention
The invention relates generally to lithography methods for producing semiconductor devices.
2. Background of the Related Art
In the semiconductor field, there has been and continues to be a trend toward smaller devices and higher device densities. This trend shows no signs of abating in the future. Higher densities require the reducing of device sizes and/or distance between devices on semiconductor wafers. In addition, the overall size and/or spacing of interconnects between devices may also need to be reduced.
The trend towards higher device densities leads to a requirement for increased resolution in lithographic process, such as photolithographic processes. Generally speaking, lithography involves any of a variety of processes for transferring patterns between various types of media. With particular regard to semiconductor fabrication processes, a silicon or other semiconductor material may be coated with a resist material that is sensitive to a particular type of radiation. Such coating may be done, for example, by spin coating of the resist material onto the semiconductor wafer. After suitable preparation, a radiation source, such as a light source or source for other types of suitable radiation, may be used to expose selected areas of the resist to radiation. The exposure pattern may be transferred through an intervening mask or reticle, such that the pattern on the mask or reticle is transferred to the resist, either positively or negatively. Exposure to light or other radiation selectively changes the properties of the resist layer during the exposure process. This change in properties can be utilized to selectively expose portions of the underlying semiconductor substrate. For example, exposure may change the solubility of portions of the photoresist to a given solvent. The solvent may then be used to wash away either the exposed or unexposed portions of the photoresist (depending on the type of solvent and the change in solubility caused by exposure to suitable radiation). Thus, the pattern of the mask or reticle may be duplicated, either positively or negatively, in the resist on top of the semiconductor substrate. Suitable operations may be then carried out on the exposed portions of the substrate, such as doping or etching operations.
A prior-art pass-through lithographic system 80 schematically is illustrated in FIG. 1. The system 80 includes a light source 82, and a pass-though mask 84 for selectively passing light 86 to a target 88, which may for example be a resist-coated semiconductor device. The pattern from the mask 84 is thus transferred to the target 88. It will be appreciated that the system 80 may include additional elements.
The system 80 allows the light 86 to proceed in a direct line from the light source 82 to the target 88, with the light 86 passing through the mask 84 substantially normally incident to the mask 84. FIGS. 2 and 3 represent light from the source 82 in a two-dimensional frequency space, showing sine of an incident angle (relative to a direction normal to the mask 84) divided by numerical aperture, in both x- and y-directions. An ideal light source of infinitesimal extent would be a point at the origin. However, an actual source of some finite size involves some light that is at a non-zero angle of incidence (not exactly perpendicular). The shape of the light from the source may be a circle 90 centered about the origin (FIG. 2) or an annular shape 92 centered about the origin (FIG. 3).
However, with decreasing feature sizes, resolution requirements have increased to the point that optical systems may no longer be able to achieve the required resolution, due to limits inherently related to the wavelengths of optical light employed in such systems. One possibility of increasing resolution beyond the limits inherent to optical photolithography systems is to utilize shorter-wave length radiation. One specific possibility has been that the use of extreme ultraviolet (EUV) radiation, having wave lengths in the range of about 30 to 700 Angstroms (3-70 nm). Use of EUV radiation allows the possibility of achieving better resolution than in optical photolithography systems. A schematic diagram of a typical EUV lithography system is shown in FIG. 4. The system 100 shown in FIG. 4 generates an image onto a target 102, such as a semiconductor substrate coated with an appropriate resist, from a reflective mask or reticle 104. The transferred pattern may involve a pattern for fabrication directly onto the semiconductor substrate, such as by doping or etching. Alternatively, the pattern may involve other semiconductor fabrication operations, such as fabrication of interconnects on a suitable pattern, for example, to suitably connect together semiconductor devices on the substrate.
A radiation source 108, for example, being a partially-coherent laser source, generates suitable EUV radiation 109, for example, having a wavelength of about 3 nm to about 70 nm. A condenser 110 may aid in directing the EUV radiation in a desired direction. Optical filtering elements 112, such as lenses or other elements, may also be used in creating a beam of radiation of a suitable size, with a suitable intensity. One or more beams 114 of EUV radiation then impinge upon the reticle 104. The reflective 104 reflects portions of the beams 114, corresponding to the reflective pattern on the reticle 104. The reflective light corresponds to the desired pattern to be exposed on the resist of the target 102. The reflected portion 116 of the beams 114 may then pass through other optical elements, such as mirrors 118, eventually being directed to the target 102.
One difference between the system 100 shown in FIG. 4, and optical systems, is that prior optical systems have generally utilized a mask that selectively allows light to pass through, as shown in FIG. 1, rather than employing a reflective reticle, such as the reticle 104.
It will be appreciated that the system 100 shown in FIG. 4 is merely a schematic, showing some of the components of a reflective system for patterning using EUV radiation. Other suitable components, such as lenses, slits, or the like, may be utilized in such systems. The system 100 shown in FIG. 4 may be able to achieve a feature resolution better than that which may be had from optical photolithography systems. For example, the system 100 may achieve a feature resolution of 45 nm or less.
FIGS. 5 and 6 show two versions of frequency space diagram for radiation incident on a reticle of a reflective system such as the system 100 of FIG. 4. In each the incident radiation 109 (which may be referred to herein as “light,” although it is not necessarily in the range of visible light) is offset from the origin, reflecting the fact that there must be some angle between the incident radiation and the outgoing radiation for a reflective system. Fully normal incident radiation would be reflected straight back toward its source. FIG. 5 shows an offset circle 120 of radiation, and FIG. 6 shows an offset annular shape 124 of radiation.
Although the general parameters of a reflective system for achieving high-resolution lithography have been set out, it will be appreciated that improvements are desirable in the design of a reflective mask, for example, in order to improve resolution and/or avoid unwanted effects in lithography.